LDAP Wizard: proper behaviour with anonymous bind, fixes #5753

Also means that Port and BaseDN fields are not invisible under some circumstances anymore. They are always there now, only disabled when values are determined in the background (also indicated by spinner, no change here). cc @jancborchardt 

Disabled textarea looks like an enabled textarea. Needs to be addressed in core.css, an extra PR will follow for this.

Please review @VicDeo @PVince81 @schiesbn or others